Home runs in baseballBaseball’s rarest offensive achievement is about to get more common

LOU GEHRIG, Willie Mays, Mike Schmidt. Scooter Gennett? As of last week, just 16 players in the 141-year-old history of America’s Major League Baseball (MLB) had hit four home runs in a game. Three of them (listed above) are all-time greats. On June 6th, the club added a remarkably undistinguished 17th member. Until last week, the most noticeable attribute of Mr Gennett’s career was probably his nickname: he started going by the name of his favourite Muppet Babies character at the tender age of five, in an effort to hide his true identity from a police officer. Mr Gennett’s offensive production during his five-year career has almost exactly matched the overall league average. But on one fateful Tuesday evening, he managed to accomplish what even baseball’s threecareerleaders in home runs could not: Boom. Boom. Boom. And…boom.

Mr Gennett’s performance was unfathomably good. A player who produced as much value in an entire 162-game season (roughly 0.65 wins above replacement) as he did on that one magical night would be expected to earn some $5m on the free-agent market. The game was also so out of keeping with his past performance that it should call into question a number of bedrock assumptions, both about Mr Gennett’s skill and the difficulty of doing what he did. Even though baseball is a game that typically requires large samples of performance to provide reliable evidence of a player’s skill, some achievements are so extraordinary they demonstrate “signature significance”—a term coined by Bill James, the father of modern baseball statistics, to refer to accomplishments over brief windows of time that only the greats tend to attain. One canonical example is striking out 18 opposing batters in a single game, a list populated almost entirely by superstars. Launching even a lone ultra-long home run appears to be another. Hitting four homers in a single contest is even rarer than throwing a perfect game, the crowning accomplishment for a pitcher. Is it too early to conclude that Mr Gennett must not be the bit player that we thought he was?

A first pass at a signature-significance test is simply to review the list of players who achieved the feat in question. By this measure, the four-home-run club is more impressive than pitching a “mere” no-hitter—which happens a few times a year, often by relatively unremarkable hurlers—but well short of 18-strikeout territory. Just five of the 17 four-homer players are in baseball’s Hall of Fame. Most of the others were at least well-above-average power hitters. However, neither Pat Seerey nor Mike Cameron ever exceeded the relatively modest benchmark of 30 home runs in a season. And excluding his four-homer game, the most home runs Mark Whiten ever hit in a season was 22.

Another argument against overreacting to Mr Gennett’s sublime evening is that none of his home runs were extra-long-distance, tape-measure shots, which are generally the province of the strongest of sluggers. According to HitTracker Online, a database of MLB home runs, had the balls’ path not been impeded by spectators’ seats, two of them would have traveled 424 feet (129 metres) each, just modestly above the league average for home runs of around 400 feet. The other two would have flown an even more humble 375 and 378 feet. In fact, Mr Gennett’s final shot of the day would not have even been a homer had it been hit in 12 of the 30 MLB stadiums with particularly inconvenient dimensions for its trajectory. The probability that a batter has elite power is far greater given a sample of four towering, majestic balls that sail deep into the stands than it is with a sample of four wind-aided fly balls that just squeak over the fence.

Neither of these caveats means that our estimate of Mr Gennett’s talent should remain unmoved entirely. Before the start of 2017, public statistical projectionsystems predicted he would hit a home run roughly once every 34 times he made contact. Based on the historical record, adding a single game with four homers on five batted balls to his pre-2017 performance should improve his rest-of-season forecast to one home run every 26.5 batted balls, a very large improvement of 29%. Nonetheless, Scooter Gennett—now with 29% more power!—still remains far closer to our prior concept of Mr Gennett than it does to a formidable slugger.

So if the preponderance of the evidence suggests that Mr Gennett is still more or less who we thought he was, how did he manage to launch four home runs in a single game? One necessary—though very far from sufficient—condition was that his teammates reached base enough for him to get five plate appearances. For a player who homers at the league average of around once every 32.5 plate appearances or so, a four-home-run game is about five times likelier in five plate appearances than in four (and is 14 times more probable if he is fortunate enough to get six attempts). Of the 17 four-home-run games in MLB history, only Carlos Delgado did it in the minimum four trips to the plate. Similarly, Mr Gennett also benefited from facing opposite-handed pitching—his career home run rate against right-handers is double his mark against lefties—and from getting good pitches to hit.

But the same could be said of thousands of player-games every season. The other factor that increased the chances that someone—though not Mr Gennett more than anyone else—would join the four-home-run club this year was the league-wide home-run rate. Just three years ago, MLB teams averaged just 1.72 home runs per game, the lowest clip since 1992. That figure has steadily increased of late. So far in 2017, it has reached 2.46 homers per contest, which would be the highest rate ever if sustained through the rest of the year.

Theories abound as to what has caused this shift. Recent studies demonstrate conclusively that the ball itself is not at fault. The most likely explanation is that batters have simply decided to swing harder, accepting more strikeouts (which are also at an all-time high) in exchange for obtaining harder contact when the bat does meet the ball.

Regardless of the cause, this “rising tide” of home runs does not appear to be going away anytime soon. And it lifts all boats to a striking degree: even a modest climb in the league average vastly increases the odds of seeing at least one four-home-run game. If we take all 146 hitters who qualified for the batting title when home runs hit their nadir in 2014, keep their home-run rates constant, and have them all play 150 games with an average distribution of plate appearances per game, there was a 7.6% chance that at least one of them would slug four in a single contest during that season. Repeating the process in 2017—with the league-wide home-run rate on pace for an all-time high—with the 146 hitters with the most plate appearances this year, that probability more than triples, to 24%. In other words, in the current environment, we should expect to see a four-home-run game roughly once every four years. At the 2014 home-run level, one would be expected only once every 13 seasons.

Moreover, upon further scrutiny, even the fact that the man to do it was a player like Mr Gennett is far less shocking than it initially appears. To be sure, Mr Gennett has a much lower chance of a four-home-run game than, say, a power hitter like Giancarlo Stanton: based on their career rates, Mr Stanton would be about 6,000 times more likely to accomplish the feat. However, the gap between the best and the rest appears to be shrinking slightly. In 2014, the top five players by home-run rate exceeded the league average by 134%. This year, in contrast, the top five surpass the (much higher) league average by “just” 105%.

And of course, there are far more average players like Mr Gennett than there are superstars like Mr Stanton. Given enough humdrum hitters, the sum of their low probabilities of a four-home-run game can exceed that of the small handful of sluggers with truly elite talent. For example, using 2017 year-to-date statistics, the chances that at least one of the ten hitters with the highest home-run rates would achieve a four-home-run game at some point this season were 10.2%. In contrast, the probability that at least one of the 136 batters under consideration outside the top ten—in other words, the very large group of players similar to Mr Gennett—would do so was significantly higher, at 15.4%. For now, Mr Gennett’s four-homer outburst may look like an unfathomable outlier. In the future, such achievements by a player of such modest pedigree may look just as unremarkable as, say, the no-hitter that the journeyman Edinson Vólquez threw three days before Mr Gennett′s game for the ages.

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